There are a lot of such shenanigans going on, people who blatantly flout the rules by not even bothering to stay for a day are just the minority of dumb ones.
Many more reside in grey areas - for e.g. many PMEs who buy a BTO but only stay there during the weekends as they stay with parents 5 out of 7 days. If you minus them being frequently abroad for holidays, these people likely only stayed in their BTO for less than a quarter of the time.
When their parents pass on, they simply inherit the flat they are staying at in name and then sell off the BTO for a handsome profit.